Measuring light spill at a bedroom window

To evidence light spill at a bedroom window you measure vertical illuminance in the plane of the glazing, in lux, twice: once with the offending light on and once with it off. The difference between the two readings is that source's contribution, and it is the number a council's environmental health officer or a lighting consultant will want. Everything else in a complaint is description. This one measurement turns it into evidence.

You can take it yourself with a calibrated meter. Whether it will be accepted depends on how carefully you record the conditions around it.

What vertical illuminance means

Illuminance is the light falling on a surface, measured in lux. Vertical illuminance is the light falling on a vertical plane, which for this purpose is the window itself.

It is not the same as the brightness of the light source you can see, and it is not the same as the light level inside the room. A floodlight aimed across a garden can produce a modest vertical illuminance at the window while still being intolerable to look at, because glare depends on the intensity of the source and its contrast with the surroundings. Both matter, and they are assessed separately.

The Institution of Lighting Professionals publishes guidance on reducing obtrusive light, and it gives limits for vertical illuminance at the windows of relevant properties, split by environmental zone and by whether the reading is taken before or after a curfew hour, usually 23:00.

Figures are those in the Institution's guidance note on obtrusive light. Check the current edition, and then check which zone your council applies to your street, because the zone changes the answer by an order of magnitude.

Where and how to take the measurement

Take the reading at the window of the affected room, not at the boundary and not in the garden.

1. Choose the worst-affected window and note which room it serves and which floor it is on.
2. Wait for full darkness and a dry night. Note the time, the date and the weather.
3. Hold the meter's sensor flat against the outside face of the glazing, facing outwards, so the sensor lies in the plane of the window.
4. Let the reading settle, then record it. Take three readings and keep all three rather than averaging them away.
5. Ask for the light to be switched off, or wait for a timer, and repeat the readings at the same position within a few minutes.
6. Subtract the second set from the first. That difference is the contribution from the source.
7. Photograph the meter in position, the display, and the view from the window with the light on and off.

Step five is the one people skip, and without it the reading proves nothing. A 6 lux measurement means little if the street lighting alone accounts for 5 of it.

If the source runs on a movement sensor, record how it is triggered and how long it stays on. Duration and frequency matter as much as the level.

Equipment and its limitations

An illuminance meter with a cosine-corrected, photopic-response sensor and a current calibration certificate is what a consultant would use, and what an officer is most likely to accept. Meters of that type can be hired.

Phone apps are not a substitute. The sensor is not cosine corrected, its spectral response is unknown, its position relative to the plane of the window is uncontrolled, and there is no calibration record. An app reading is useful for telling you whether a proper measurement is worth arranging, and for nothing beyond that.

Two further sources of error. Taking the reading through the glass from inside cuts the value by a proportion you cannot quantify. And a pale wall or a parked car near the window can bounce light onto the sensor, which is real light but may not be what you think you are measuring.

Record the make, model and calibration date of the meter. Without that line, the numbers carry much less weight.

Recording the conditions

The measurement is one line in a record that should also say:

• The address, the room, the floor and the window orientation.
• The date, the start and end times, and the sky conditions.
• What the source is, where it is, how high it is and where it is aimed.
• Whether the source is continuous, timed or triggered, and the pattern over an evening.
• Other light sources present, and whether they were on during both sets of readings.
• Whether curtains or blinds were open, and whether the light is visible from the bed.

Keep a diary over two or three weeks rather than relying on one night. Frequency and duration are what turn an annoyance into a nuisance in law, and a dated log of every evening the light was on after 23:00 is more persuasive than a single high reading.

Raising it as an objection or a complaint

Two routes, and they are different.

If the light comes from something with a live planning application, the effect on residential amenity is a material consideration, and an objection asking for a lighting assessment and a curfew condition is a reasonable thing to put. A Lighting Assessment prepared to the Institution's guidance is what an applicant would normally submit, and councils usually expect a suitably qualified author for it, with professional review strongly advised.

If the light comes from something already built and operating, artificial light emitted from premises can be a statutory nuisance under Part III of the Environmental Protection Act 1990, a limb added by the Clean Neighbourhoods and Environment Act 2005. The council investigates, and where it is satisfied a statutory nuisance exists it has a duty to serve an abatement notice.

Certain premises are exempt from that limb, including airports, harbours, railway and tramway premises, bus stations, vehicle operating centres, lighthouses and prisons. Outdoor sports facilities have the benefit of a best practicable means defence. And street lighting on the highway is generally outside the regime altogether, which means the route there is the highway authority rather than environmental health.

Planning permission is not a defence to statutory nuisance, which is worth understanding properly before you accept that a consented scheme cannot be touched. The same tension appears with statutory nuisance after planning permission is granted and with servicing and delivery noise at night.

What the council can require

Under the nuisance route, an abatement notice can require the light to be abated, restrict when it operates, or require specified works such as re-aiming, shielding, reducing output or fitting a timer. Contravening the notice without reasonable excuse is an offence.

Under the planning route, where a lighting condition has been breached the remedy is planning enforcement, which is slower and discretionary.

In practice the fix is usually small. Re-aiming a luminaire so its beam stops at the boundary, fitting a shield or louvre, lowering the mounting height, reducing the lamp output, or setting a timer to switch off at a curfew hour resolves most domestic and small commercial cases without anyone going to court. Reflected light from a specular surface is a separate problem again, closer to the work described for glint and glare near an aerodrome.

If you want to see what an applicant is expected to submit on lighting and similar effects, the index of report types sets out the documents councils commonly ask for. This is general guidance, not legal advice; check the position with your local authority or a qualified professional before relying on it.